This invention relates to data measuring of downhole conditions within wells during drilling and more particularly relates to apparatus and methods for telemetering data in such operations using an acoustic signal transmitted through the drilling fluid during drilling.
Various logging-while-drilling techniques for telemetering data representing downhole conditions during drilling of a well have been suggested. One approach uses a technique which imparts an acoustic signal, modulated according to the sensed conditions, to the drilling fluid, i.e., the drilling mud, for transmission to the entrance of the well where it is received and decoded by uphole electronics circuitry. This basic technique is described in detail in U.S. Pat. No. 3,309,656, issued Mar. 14, 1967 to Godbey entitled "Logging-While-Drilling System." In this system the modulated signal is applied to the drilling fluid using an acoustic signal generator which includes a movable member for selectively interrupting the drilling fluid. At least part of the flow of the drilling fluid is through the acoustic generator, and the movable member selectively impedes this flow, transmitting a continuous acoustic wave uphole within the drilling fluid.
The acoustic signal is preferably phase shift keyed modulated, as disclosed in U.S. Pat. No. 3,789,355, issued Jan. 29, 1974, to Patton entitled "Method and Apparatus For Logging While Drilling." According to phase shift keyed (PSK) modulation, the data derived in response to the sensed downhole condition is initially encoded into binary format, and the acoustic signal generator is driven at speeds so that the phase of a constant frequency carrier wave generated in the drilling fluid is indicative of the data. In particular, a non-return to zero type PSK mode is used wherein the phase of the carrier signal is changed only upon each receipt of data of a predetermined value. For example, for data encoded in binary, the phase of the carrier wave may be changed for an occurrence of a logic 1 data bit.
Ideally the phase change of the carrier signal would be instantaneous upon occurrence of the data of the particular value. This is because the downhole telemetering unit is continuously transmitting data to the uphole receiving instruments where the data in turn is continuously decoded. Any delays in effecting the phase change and in returning the acoustic signal to its carrier frequency introduce errors and/or inefficiencies into the system.
As a practical matter, however, the phase of the acoustic signal cannot be changed instantaneously in response to data of the predetermined value. Inherent delays are introduced by the physics of the system. The motor control circuitry which operates the motor-driven acoustic generator is adjusted accordingly to effect optimum response of the generator. Past proposals, such as the above-referenced Godbey and Patton patent, and in U.S. Pat. No. 3,820,063, issued June 25, 1974, to Sexton et al. entitled "Logging While Drilling Encoder," have proposed several circuits for implementing the motor control circuitry. In the Patton and Sexton et al. patents, the speed of the motor was to be temporarily varied such that, upon returning of the motor speed back to the carrier frequency producing speed, the desired amount of phase change would be accumulated. In the Sexton et al. patent, this was accomplished by varying the speed of the motor in a first direction until a predetermined amount of phase shift had been accumulated. The motor speed was then returned in the other direction to the carrier frequency producing speed for a pretermined duration of time, thereby attempting to accumulate the remainder of the desired amount of the phase change.
The above proposals lacked preciseness in returning the speed of the acoustic generator drive motor to the constant carrier frequency producing speed (the carrier speed) during the phase changing (during modulation). The proposals appeared to suggest tuning of the respective systems such that the return approximated the accumulating of the desired amount of change and approximated terminating the return when the speed of the motor had reached the carrier speed. The proposals, however, failed to detect the actual speed of the motor which would allow termination of the return precisely upon reaching the carrier speed. In failing to detect the actual motor speed, the proposals failed in providing a system which would allow the return to be in the shortest possible period of time; i.e., failed in providing a system which would allow the driving of the drive motor at maximum excitation yet which would obviate undershoot or overshoot of the carrier speed. The proposals relied on a separate phase and frequency adjusting and maintaining circuitry to adjust the phase and frequency to the proper values after approximate return to carrier speed to account for the undershoot and overshoot. Such adjusting and maintaining circuitry, however, required a relatively long time to change the motor speed any substantial amount, thereby failing to minimize the period of the return. By failing to minimize the period of the return, the proposals either allowed inaccuracies to be introduced into the system or provided an unnecessarily slow encoding/data transmission system.
More specifically, in the system proposed in the Sexton et al. patent, the speed was returned by applying a predetermined level of excitation of the drive motor for a fixed, predetermined duration of time. After expiration of the predetermined duration of time, control of the motor speed was returned to the phase and frequency adjusting and maintaining circuitry, regardless of the total amount of phase accumulated or of the actual speed of the drive motor.